Poly(hydroxyalkanoic acid) (PHA) such as polylactic acid (PLA) is a resin comprising renewable monomer such as production by bacterial fermentation processes or isolated from plant matter that include corn, sugar beets, or sweet potatoes. The resin can be used for thermoformed packaging articles such as cups, trays, and clam shells. Generally, the resin is first extruded into an amorphous sheet and formed above is glass transition temperature (Tg) into finished articles.
Unoriented sections of the articles do not fully crystallize because many PLA grades crystallizes too slowly in high speed thermoforming equipment or crystallize with less than 10% crystallinity. As PLA grades popular for thermoforming have a Tg of about 55° C., articles of such PLA that are thermoformed into cool molds have poor dimensional stabilities when heated above the Tg. A thermoformed or stretched article may shrink in a few seconds more than 5% (sometimes 50%) when heated above the Tg. The tendency for shrinkage is especially high (to 50%) in those parts of a molded article that experience a critical amount of orientation less than about 50% (final length or area is 50% greater than the pre-formed length or area) but more than about 10%. Those regions having higher than about 50% orientation may experience some strain-induced crystallization and thereby have shrinkages as low as 10% at temperatures slightly above Tg. Those areas having no orientation may have low shrinkages (<10%); however, these areas are soft and easily deform at temperatures slightly above Tg. Those regions in between 50% and 10% have the highest shrinkage which is the subject of this application. High forces can be generated by shrinkage and therefore the shrinkage of one region in a complex hollow article can be magnified into a larger dimensional effect on overall structure. Therefore for the purpose of this application the desirable shrinkage is less than 10%, less than 5%, or less than 1%.
The shrinkage force is due to the presence of amorphous orientation which is PHA containing stretched PHA molecules not crystallized and amorphous but immobilized in place by the rapid cooling in the molded article to below its Tg. When the temperature rises above Tg these stretched amorphous molecules relax rapidly and induce or cause shrinkage if the article is not constrained from shrinking. Some additional shrinkage in a few minutes can arise from crystallization if the PHA is a particularly fast crystallizing PHA. For example of a fast crystallizing PHA is PLA having molecular weight below about 10,000 Dalton, low D-lactide (meso-lactide) content, and/or use of high amounts of special nucleators and/or if the temperature rises to half way between Tg and the melt point.
To solve the problems, one may increase crystallinity or decrease amorphous orientation or use a combination.
For example, to increase the crystallinity, one may heat-treat the molded article at a temperature half way between the glass transition and the crystalline melt point. For typical PLA that would be 110° C. for several seconds to avoid the shrinkage when heated above 55° C. Doing so will cause the article to shrink in the first few seconds of the heat treatment. To compensate for shrinkage, the thermoforming mold would have to be designed with a larger size so that the shrinkage of the article is counter balanced. Such a mold design is complicated and nevertheless given inconsistent results due to small variations of the molding process and sheet causing larger effects on shrinkage.
One may heat-treat the article for several minutes at about 110° C. while it is constrained from moving in the mold. Doing so would extend the overall thermoforming cycle time too much.
One may heat-treat the article for several seconds at about 110° C. while it is constrained from moving in the mold. Doing so would leave as amorphous those regions of the article that have not been oriented. Removal of the article from the hot mold would cause deformation of those regions.
Alternatively, one may reduce the amorphous orientation, an article may be thermoformed at a high temperature, above the half-way temperature between Tg and melt point. At excessively high temperatures such as approaching the melt point would give high sagging of the hot sheet or deformation at its supports and poorly shaped thermoformed articles. Slightly lower temperatures could be problematical due to exudation of oligomer or additives on the surface of the mold giving surface roughness to the molded article. Running at normal thermoforming speeds and about 20° C. above the half-way temperature between Tg and melting temperature (not excessive) gives a molded article having few stretched amorphous PHA molecules and gives reduced shrinkage compared with a molded article having been stretched at a temperature half-way between Tg and the melt point. However, the article will be 90% or more amorphous, which is very soft and deforms easily above the Tg, while an article of >10% crystalline is generally desired. A 100% amorphous article may also experience some shrinkage when held for several hours at temperatures around the Tg due to some beginnings of crystallization or other molecular re-arrangements.
Alternatively an article can be made such that the resin is stretched during thermoforming to more than about 200%. Doing so may give clarity and dimensional stability due to strain-induced crystallization process, but this large amount of stretching limits the shape of molded articles to those that are very long and narrow.
One may also increase the crystallinity or rate of crystallization by use of a nucleator for PHA. Many nucleators exist such as particles that are solids at the processing temperature of the PHA and are dispersed to sizes less than about 50-micron. Such particles include naturally occurring minerals especially non-abrasive minerals such as muscovite, montmorillonite, and talcite; other inorganic solids such as nitrides of silicon or boron. Using nucleator introduces haze or opacity to the otherwise transparent PLA articles thereby impairing the value of the articles. See, e.g., U.S. Pat. No. 6,114,495, U.S. Pat. No. 6,417,294, and WO 03014224.
Therefore, there is a need to produce a clear article from PHA and to increase the dimensional stability of the clear article.